Its a shame but the reality is that the worst affected for both autism and Downs aren't really people you'll encounter in public. So of course people are skewed to the ones who are able to function at least a bit normally.
When I was a kid, a friends sister had very severe autism. By the age of 8 she couldn't walk very well, couldn't talk,had to still be hand fed puree, was fully incontinent, and was completely non communicative except for screaming and hitting/kicking/biting. Her parents and sisters had all sustained bloody injuries trying to assist her during meltdowns and it was obviously not a great situation for her siblings. Luckily the family were much better off financially than mine, cos they managed to send the healthy siblings off to boarding school to keep them from being psychologically damaged by their completely disabled sister. And the parents weren't neglectful, the sister got so many therapies and they poured in so much time trying to improve her communication and mood issues but it wasn't possible. That poor girl is about 25 now, she never improved on any metric, and now is full time in a facility because she was able to really injure her parents as they got older. They've dumped their entire retirement and savings into a trust so that the sister can stay with trained staff.
A family friend has a young son with Downs. Downs can really fuck up muscle tone, it's why they all have that similar kind of body shape and not great coordination. This poor little boy is 4 and his throat and tongue is so weak he can't swallow at all and can't speak. He has to be tube fed via a port in his stomach because he will choke if he eats. He can just barely manage a sippy cup of water to keep his mouth from drying out. He's recieving speech therapy but he can't make any sounds that are clear. It's to be seen how his cognitive standards are, they're now using sign language and flash cards for him to communicate. He could be anywhere on the spectrum of IQ for DS kids, it's just the physical barrier to communicate with him. Seems like he does understand a lot of what his parents say, he just can't speak back. But it's obvious that he's gonna have lifelong medical issues, especially if he develops any kind of airway issues because his tongue and throat are so weak.
This is why I roll my eyes so hard when people who really have never seen the worst outcomes start preaching about how "we shouldn't even call them disabled! They're special / unique / different!" Like fuck no. Even the least severe of thlse conditions are still disabled compared to normal society, they do need assistance. And the ones who are really badly affected, they get completely ignored because nobody wants to face the reality of it. For every feelgood story about a cashier at Walmart with downs, there are several who can't take care of their basic needs without help. For every autistic savant, there is a kid who can't interact with the world at all.
It's a sad thing that nobody wants to acknowledge because what's the solution? There isn't one. They need expensive full time care from trained staff and nobody has a good solution for how to manage that outside of the family bankrupting themselves to self fund it, or having a healthy member spend all their time to care for them